Nanotubes Grow Smaller than a Nanometer

Shrinking nanotubes

As it is, nanotubes are tiny. But researchers at the University of California, Berkeley, have found a way to make these carbon structures even smaller. The size of the tubes, which are used in mass and chemical sensors as well as transistors and oscillators, imparts properties that allow for more exacting performance--for instance, the ability to oscillate at higher frequencies or improved conductivity. "We now have control of an important nanoscale building block, which we can now incorporate into all kinds of devices," says Tom Yuzvinsky, a condensed matter physics graduate student, who is a co-author of the study.

Previous attempts to shrink nanotubes involved irradiating the tiny structures with electrons. This bombardment would cause defects due to mass loss--turning the smooth pipe effectively into Swiss cheese, Yuzvinsky explains. In the new method, the Berkeley researchers wanted to take advantage of this mass loss, so they heated a tube by "pumping tons of current through it," according to Yuzvinsky, while simultaneously irradiating it. The amount of heat going through the nanotube--its temperature rising to thousands of degrees--nearly melts it into a liquid. Although defects are made in the walls of the structure, the atoms, being in a near-liquid state, rearrange to fill the spaces in a process called annealing. "You're getting to a point when it can rearrange into its most stable shape," Yuzvinsky says, "which is its strongest one." The electrical contacts at either end of the nanotube are fixed and unharmed, and the scientists monitor progress using a transmitting electron microscope.

In the paper, which is slated for publication in the December 13 issue of American Chemical Society's Nano Letters, the authors describe the shrinking of one nanotube from 21.5 nanometers in diameter to 0.9 nanometer. "For some applications," Yuzvinsky notes, "you need to set the [nanotube's] diameter exactly. So, if you have a mechanical oscillator, and you want to adjust the frequency it's operating at, you need to adjust the stiffness of the nanotube that's oscillating. And one easy way to do that is to change the diameter."

The length of time the process takes depends on the amount of current used and the irradiation time, Yuzvinsky notes, adding that the new paper describes shrinking a nanotube in 30 minutes, although the process could go faster or slower. P. M. Ajayan, a materials engineer at Rensselaer Polytechnic Institute, says he is impressed that the Berkeley team could manipulate nanotubes while taking "direct measurements during the structures' transformation. He fears, however, that "the process may not be feasible for real device engineering since electron irradiation at high voltages is not compatible with fabrication processes."

Currently, this new method only reduces the size of nanotubes one at a time, but Yuzvinsky says the process could be automated. Besides tweaking the method for mass production, he states that, in its current form, "the next logical step is to apply it all over the place." For example, he says shrunken nanotubes could be employed to create custom-made sensors for detecting mass on subatomic scales. He also points out that the process will allow more work to be done on the physics of nanotubes. "Now you can select the diameter beforehand and then measure electronic properties, mechanical properties, chemical properties--whatever you want to do."

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